Heating unit having a ptc heating resistor

ABSTRACT

The invention relates to an electrical heating unit for appliance such as coffee makers, hot plates and the like. The heating unit includes a PTC resistor which is mounted so as to be completely surrounded by a liquid medium so that a uniform temperature is maintained for the external surface of the resistor. This uniform temperature prevents cracking of the resistor due to unevenly distributed thermal strains.

United States Patent my Abildtrup HEATING NIT'IIAVING A PTC HEATING RESISTOR I Denmark [73] Assignee: Danfoss A/Sl, Nordborg,'Denmark [22] Filed: July 21, 1971 211 Appl.No.: 164,627 9 [30] I Foreign Application Priority Data 9 Aug. 14, 1970 Germany ..P 20 40 523.6

[451 March 6, 1973 2,236,837 4/1941 Rimmel ..219/440 X 3,029,331 4/1962 Kowarik ..338/22 R X 3,044,968 7/ 1962 lchikawa ..252/520 3,073,940 1/1963 Brandl ..219/438 X 3,214,719 10/1965 Turner ..338/22 R 3,274,360 .9/1966 Brand] ..219/331 X 3,489,884 1/1970 Waseleski, Jr.. ....338/22 R X 3,51 1,970 5/ 1970 Kjellberg .219] 331 3,551,644 12/1970 Sano et a1 ..219/441 FOREIGN PATENTS OR APPUCATIONS 258,804 5/ 1949 Switzerland ..219/439 470,827 8/ 1937 Great Britain .,.2 1 9/442 Primary Examiner-Volod'ymyr Y. Mayewsky Attorney-Wayne B. Easton 1-v ABSTRACT mounted so as to be completely surrounded by aliquid medium so that a uniform temperature is maintained for the external surface of the resistor. This uniformtemperature prevents cracking of the resistor due to unevenly distributed thermal strains.

1 Claim, 2 Drawing Figures HEATING UNIT HAVING A PTC HEATING RESISTOR The invention I relates to a heating or regulating device comprising a FTC-resistor.

Resistors, the resistance of which increases with temperature (FTC-resistors), are frequently used in industry, for example as temperature-responsive resistors, as the starting resistors of single-phase asynchronous mo- I tors, or as heating resistors. in all cases, their currentliquid, e.g., coffee. For this purpose, one or more PTC- V resistor bodies in disc form are enclosed in a fluid-tight manner in a casing,-tw faces of the casing being in heat-conducting contact with the faces of the discs, and an electrically insulating layer of mica being fitted between the faces of the casing and the discs. In an arrangement of this kind, the current-carrying capacity of the FTC-resistors is still lower.-

The object of the invention is to provide a heating device comprising a FTC-resistor, which device can be more heavily loaded than previously as regards electrical power or temperature.

According tothe invention, this object is achieved by completely immersing the FTC-resistor in a fluid which undergoes a flow movement when differences in the surface temperature of the FTC-body occur.

The fluid surrounding the FTC-resistor is a uniform good conductor of heat, since the fluid is kept continuously in circulation, and as long as the PTC-resistor is heated, the fluid is warmed on the PTC-body and cooled on the case. A heating element of this kind can be sealed off in a fluid-tight manner and can be used for heating a fluid. It can also be incorporated in a hotplate or attached to its underside.

substance which. is also kept in circulation by differences in temperature. Consequently, all the surface zones of the FTC-body are at approximately the same temperature at each moment. When the surface temperature is uniform, the danger of cracking due to ther- 'mal stresses is very much reduced, since cracking of this kind is usually initiated at the surface of the body.

Tests have shown that a FTC-resistor, known per se, can in this way be loaded several times more heavily; for example, a FTC-resistor that can normally carry only 25 W can now carry 225 W. Furthermore, it is possible to keep the PTC-resistor at a point on the straight part of the characteristic curve, whereas hitherto the working point often moved to the upper or lower terminal point.

Expediently, the fluid is not electrically conductive. Then the FTC-resistor can be directly immersed in the fluid.

If however a' conductive fluid is used, the FTC-body should be provided with a coating of electrically insulating material. In this connection, it suffices to apply very thin coatings that neither adversely affect the thermal conductivity nor apply any forces at all to the PTC- body.

In a preferred embodiment, the FTC-body and the fluid are placed in a closed case. Since the FTC-body is completely surrounded by fluid, its surface is virtually not in contact with this case at any point. Nevertheless,

- a heating element is obtainedthat can be used in large instead of this, the PTC-body can also be fitted directly in a fluid-medium that is to, be heated. Thus, the fluid to be heated, e.g., water or coffee, can be used directly for evening out the surface temperature. In some instances the FTC-resistor must be provided with a thin, electrically insulating protective coating, e.g., of glass or plastics material, in order to meet safety requirements. This arrangement is also suitable when a FTC-heating resistor is to be fitted directly in the water.

compartment of a washing machine.

When a case is used for the FTC-body and the fluid,

it is expedient for the case to be provided with a device for compensating the differences in the volume of the fluid governed by temperature. A device of this kindcan be agas-cushion, a resilient wall or the like. It is then unnecessary to ensure'that the fluid has the smallest possible coefficient of thermal expansion.

In all the applications of. the heating device in accordance with the invention, the FTC-resistor is not only used to provide a predetermined amount of heat;

it is also enabled to keep the temperature of the medi um to be heated at a constant level, since it automatically reduces the supply of power when the temperature rises.

The invention will now be described in more detail by reference to embodiments illustrated in the drawing, in which:

FIG. I is a schematic section through a hot-plate constructed in accordance with the invention, and

FIG. 2 is a schematic section through a water heater constructed in accordance with the invention.

Referring to FIG. l,'a PTC-body l andthe oil 2 surrounding it are contained in a metal case 3. Leads 4 and 5 run through lead-through passages in insulations 6 and 7 to the end-faces of the body 1. The top of the metal case 3 is in heat-conducting contact with the bottom of a metal plate 8, on whicha cooking vessel for example can be placed. The bottom of the case 3 contains an opening 9 which is covered by a resilient sheetmetal element 10.

When current is passed to the PTC-body 1, it and the surrounding oil 2 are heated up. Flow movement is imparted to the oil which passes heat to the case 3. Apart from that portion lost by radiation to the surrounding air, this heat is passed to the metal plate 8. The oil 2 ensures that the entire surface of the body 1 is at the same temperature at any moment. Changesin volume of the oil 2 are compensated by deflection of the sheet-metal part 10. When the heating device has acquired a predetermined temperature, the resistance of the body 1 will have risen to an extent such that the power supplied is reduced. The hot-plate is thenenabled to maintain this predetermined temperature at a substantially constant level. After the system has been switched off, the entire arrangement gradually cools down, and the entire surface of the body [again has substantially the same temperature at any moment.

In the arrangement shown in FIG. 2, a FTC-body 13, having the form of an annular disc, is fitted in a con tainer l2 filled with water 11. The PTC-body has a thin electrode coating 14 and 15 on each of its two faces, and'leads l6 and 17 are secured to these coatings and pass through lead-through passages in insulations 18 and 19 upwards through the base of the container. The FTC-body l3 and electrodes 14 and 15 are surrounded by a thin electrically insulating coating 20 which, as in the case of the electrodes 14 and 15, possess good thermal conductivity and apply no external stresses to the body 13.

In this construction, the water 11 that is to be heated is used for keeping the surface temperature of the PTC- body 13 uniform. Provision should therefore be made for the body 13 also to be covered by the water during the cooling period. This can be achieved for example by fitting a water-intake cock 21, incorporating a valve 22, in the wall of the container and at a point just above the FTC-body 13.

If the principle illustrated in FIG. 2 is applied to a washing machine, the PTC-body can be fitted in a recess in the base, in which recess residual water is always present.

I claim:

1. A heating unit comprising a sealed metal case for a liquid, said case having top, sides and bottom walls, a PTC heating resistor mounted in said case in spaced relation to said walls, a heat conductive liquid in said case in substantially complete surrounding relation to said resistor, said top wall being in contact with a heat conductive state for conducting heat thereto, electrical leads for said resistor extending through one of said walls and electrically insulated therefrom, said one of walls having an opening, and a resilient sheet portion closing said opening and being outwardly expandable in response to the liquid inner pressure applied thereto. 

1. A heating unit comprising a sealed metal case for a liquid, said case having top, sides and bottom walls, a PTC heating resistor mounted in said case in spaced relation to said walls, a heat conductive liquid in said case in substantially complete surrounding relation to said resistor, said top wall being in contact with a heat conductive state for conducting heat thereto, electrical leads for said resistor extending through one of said walls and electrically insulated therefrom, said one of walls having an opening, and a resilient sheet portion closing said opening and being outwardly expandable in response to the liquid inner pressure applied thereto. 